Fuel Pressure Question

[CFI1513840]

I have an AFP high pressure boost pump with a TMXIO-360. I just performed a fuel flow and pressure test of the boost pump. With the fuel line disconnected at the throttle body and routed into a container, I calculated the flow rate at about 45 GPH. With the fuel line reconnected, throttle closed, and mixture set at idle/cutoff, I get a fuel pressure of about 60 PSI.

I thought these readings were kind of high and wondered if they compare favorably with what other people are getting.

[Noah]

Your open-system fuel flow sounds about right, but 60 psi when on relief at zero flow is definitely high - the relief valve has a 25 psi design cracking pressure. I get about 27 psi when the boost pump is "on relief" - meaning the relief valve is open allowing flow to circulate back upstream of the pump since it has nowhere to go. I would call Airflow and talk to Don. You might have a bad relief valve or some blockage in the relief line which takes that flow back upstream of the pump.

Or maybe you have not finished all of the plumbing yet and just put a cap on the relief valve outlet? This is called "deadheading" and you can damage the pump if operated without a relief valve. I don't believe that these pumps are designed to be operated at zero flow. The pump ASSEMBLY is designed to be operated at zero flow because the ASSEMBLY contains a relief valve which allows flow throught the pump to keep it cool and to keep pressures lower.

[CFI1513840]

Noah,

Thanks for your input. I have the relief valve plumbed back to the fuel selector output tee according to the installation instructions. I can't imagine how the pressure could reach 60 PSI downstream if the relief valve is operating properly and your response confirms this. I have confidence in the pressure sensor as I tested it during the Dynon installation with air pressure at about 30 PSI if I remember correctly.



I sent a message (and pictures of the installation) to AFP asking for advice. I'm hoping it might be a simple matter of replacing the relief valve.

[CFI1513840]

I talked to Don at AFP and he gave me instructions on how to test the system. Instead of disconnecting the input fuel line at the throttle body, he had me disconnect the inlet line at the flow divider. The line was then routed to a container and with the throttle open and full rich mixture, the pressure stabilized at 31 PSI. After moving the mixture to idle/cut-off the pressure remained constant at 31 PSI, so everything looks okay.

From what I understand, the bypass valves in the engine driven pump cause the pressure to backup on the downstream side of the boost pump, therefore the relief valve never sees the necessary 25 PSI differential needed to open and regulate the pressure. This can't happen when the engine is running, so it's really a non-problem.

Great support from Don at Air Flow Performance!